A ship speed indicator has been proposed in U.S. Pat. No. 3,898,878 (Stallwood and Hartley) for measuring ship's speed by calculating the time taken for acoustic signals to travel between two projector-receiver modules, each containing a transmitter and a receiver, positioned at preselected points on the external bottom surface of the ships hull. A logic and display module located in the interior of the ship is connected between the projector-receiver modules. Measurements are made of the time taken by acoustic signals to travel from the projector of one of the modules to the receiver of the other and vice versa. From the distance between the selected points and the difference in time taken for the acoustic signals to travel in the opposed directions between the modules, the speed of the ship is derived.
For this system, the frequency of the acoustic signal projected into the water is 100 kilohertz. This frequency of propogation allows signal analysis to be conveniently performed. However, this choice of frequency has several disadvantages. Firstly, the frequency is used by conventional sonar devices and echo sounders. It also has a tendency to propogate large distances. The signals received by the respective modules are therefore prone to interference. Furthermore, in order to gain the required resolution at this frequency, a large distance required between the modules (approx. 10 meters) and, hence, the system is unsuitable for use with small ships or boats.